1. Field of the Invention
This invention relates to a print tone measuring method.
2. Description of the Related Art
In order to perform proper printing with a printing machine, it is necessary to control ink feeding rates properly. For controlling the ink feeding rates, it has been conventional practice to measure densities of control strips with a densitometer and determine from density data whether the ink feeding rates are proper or not. However, the density data from the control strips alone is not necessarily sufficient for attaining a proper color tone and the like for a picture area.
For this reason, a print tone measuring apparatus is used which provides control data for controlling the ink feeding rates of a printing machine. The control data is produced by comparing an image on reference paper and an image on an actual print.
The reference paper is also called proof paper, and serves as a reference indicating a color tone of finished prints to obtain proper prints. Printing paper actually printed is also called sampling paper which is extracted by the operator from a discharge station of a printing machine at certain intervals during a printing operation. The printing is considered proper when the color tone of the sampling paper substantially coincides with the color tone of the reference paper.
Where such a print tone measuring apparatus is used, it is difficult to attain an agreement in color tone and the like in all areas between the image on the reference paper and the image on the actual print. For this reason, use is made of control points such as representative points having colors that characterize a picture pattern on the print. These control points are key points used in order to control the ink feeding rates, and are set to the image by the operator.
The control points must be set to accurately corresponding positions on the image of the reference paper and the image of the actual print. Japanese Unexamined Patent Publication No. 10-305562 (1998) discloses a print evaluating apparatus, in which a pair of reference points are specified on each of an image of reference paper and an image of an actual print for indicating image positions, and positions of control points on the image of the actual print are calculated from these reference points and the control points on the image of the reference paper.
With the above method, however, a partial distortion of the image of the print makes it impossible to attain an accurate corresponding relationship between the control points on the image of the reference paper and those on the image of the print. That is, a curvature aberration of an image pickup optical system for reading images from the reference paper and print, or an error due to a distortion of printing paper, results in a displacement between the control points on the image of the reference paper and those on the image of the print. Such a displacement hampers an accurate measurement of the print tone.
An image area usually is formed of halftone dots. Particularly where large halftone dots are used, a disagreement may occur between color information on a single pixel and color information on an image formed of a plurality of halftone dots. Noise of a CCD camera acting as an image pickup device, or dust or scratches on the print, may also hamper an accurate measurement based on color information on a single pixel.